1. Field of the Invention
This invention relates to an apparatus and method for image processing and apparatus for coding that can define a quantizing parameter in a suitable way.
2. Discussion of the Background
Recently, apparatuses conforming to the MPEG (Moving Picture Experts Group) scheme to handle image data are becoming more widespread. In such apparatuses compression is made by orthogonal transform such as discrete cosine transform and motion compensation through utilizing redundancy unique to image information for the purpose of efficient transmission and storage of information, both in distributing information at the broadcast station and in receiving information at the general household.
Particularly, MPEG2 (ISO/IEC13818-2) is defined as a general-purpose image coding scheme. This is the standard covering both images of interlaced scanning and progressive scanning as well as both images of standard resolution and high definition, now being broadly used over a wide range of applications for professional and consumer uses.
The use of the MPEG2 compression scheme can realize high compression efficiency and favorable image quality by assigning a code amount (bit rate), for example, of 4-8 Mbps for a standard-resolution interlaced scanning image having 720×480 pixels or of 18-22 Mbps for a high-resolution interlaced scanning image having 1920×1080 pixels.
MPEG2 is mainly for high image-quality coding adapted for broadcast, not suited for the lower code amount (bit rate) than MPEG1, namely the higher compressive coding scheme. The need for such a coding scheme is expected to expand in the future, due to the spread of personal digital assistants. In order to cope with this, standardization has been completed on the MPEG4 coding scheme. Concerning the image coding scheme, the standard has been approved as an international standard ISO/IEC14496-2, December 1998.
Furthermore, another standardization, called standard H.26L (ITU-T Q6/16 VCEG), has recently been pushed forward aiming at image coding initially for TV conferences. H.26L is known to realize higher coding efficiency despite requiring a much greater operation amount in coding and decoding as compared to the traditional coding schemes such as MPEG2 and MPEG4. Meanwhile, Joint Model of Enhanced-Compression Video Coding is now under standardization as part of MPEG4 activity, to introduce functions not supported under the H.26L standard on the basis of the H.26L standards, and to thereby realize higher coding efficiency.
The coding apparatuses under the MPEG and H.26L standards realize efficient coding by making most of the local pieces of information of an image.
An image has a nature that a complicated part of the image, even if coded coarser in quantization than other parts, can be less visually recognized of image deterioration.
For this reason, in the foregoing coding apparatus, the image is divided into a plurality of parts to detect a complexity of the image on each part. Based on the detection result, complicated parts of the image are quantized coarsely, while the other parts are quantized finely, thereby reducing a data amount while suppressing the effect of image deterioration.
The information about image complexity is called activity.
In the foregoing coding apparatus, activity is computed on the image data as a subject of quantization, to generate a quantizing parameter to regulate a quantizing scale on the basis of the activity.
In the meanwhile, the foregoing H.26L standard defines (Periodic Quantization) to quantize the image data of motion picture by increasing coarseness 1.12 times (12% increase) with increasing 1 in a quantizing parameter.
Accordingly, taking account of this, the present inventors recognized there is a necessity to generate the quantizing parameter on the basis of the activity.